This invention relates to systems for soldering electrical and electronic components onto substrate boards, and more particularly to an improved apparatus and method for mass soldering electrical and electronic components to the top and bottom surfaces of substrate circuit boards or the like in a single pass. The invention has particular application to soldering of surface mounted components such as chips or the like to both top and bottom surfaces of printed circuit boards and will be described in connection with such application. It will be understood, however, that the invention also may be used for soldering circuit boards having only conventional through-hole mounted components on the top surface alone, surface mounted components on the bottom surface alone or the combination of the two, and circuit boards having both conventional through-hole mounted components and surface mounted components on the top surface and surface mounted components on the bottom surface.
In response to recent demands for compact electronic appliances and instruments, there has been a tendency to use small, thin chip type electronic parts, i.e., surface mounted components and/or carriers therefor, for mounting on printed circuit boards in place of conventional, discrete type, leaded components. The advent of surface mounted component technology has afforded striking new levels of efficiency to the circuit board designer and, at the same time, has posed new challenges for the manufacturing engineer. Apart from the frequently noted advantages of component size and geometry, the circuit board designer has at his disposal the capability of populating both top and bottom surfaces of printed circuit boards with components.
Various techniques have been proposed by the art for mass soldering surface mounted components to a single sided circuit board. One technique involves fixturing the components to the bottom side surface of the board, e.g., using an epoxy adhesive or the like, and then engaging the bottom surface of the board and fixtured components with molten solder by passing the board bottom surface and fixtured components in contact with a body of molten solder as, for example, through the crest of a standing solder wave. Such a process is described in U.S. Pat. No. 4,465,219 to Kenshi Kondo.
It has also been proposed to solder surface mounted components to the top surface of a circuit board using a solder cream or paste, or a solder preform or the like. In such prior act technique, the surface mounted components are loaded in position on the top side surface of the board, together with suitable solder preforms or a solder cream or paste, and the circuit board and loaded components are heated to a temperature sufficient to reflow the solder. Heating to effect solder reflow can be accomplished by baking the circuit board and components in a radiant furnace or the like. Alternatively, the circuit board and components may be immersed in a vapor heated to a temperature above the melting point of the solder, in accordance with the so-called vapor phase soldering or condensation soldering process. Such a process is described in U.S. Pat. No. 3,866,307 and 4,321,031, which are given as exemplary. Radiant heating reflow soldering and vapor (condensation) soldering have achieved a certain degree of commercial acceptance although each technique has certain drawbacks. For one, the techniques are somewhat slow, and both techniques have a tendency to overheat components which could cause damage to heat sensitive components. Also radiant heating reflow soldering is susceptible to line-of-sight shadowing and thus may not be entirely satisfactory for high density applications. Furthermore, the fluid used to create the vapor for condensation soldering is quite expensive, and the thermal decomposition products of the fluid produced are dangerous to health.
While such prior art systems, in spite of the aforesaid and other disadvantages, are being used to solder surface mounted components to one surface of a circuit board, no system was heretofore available for mass soldering components populating both (opposed) surfaces of a printed circuit board in a single pass.
It is thus a primary object of the present invention to provide a mass soldering system, i.e., apparatus and process, which overcomes the aforesaid problems of the prior art.
Another object of the present invention is to provide an improved apparatus and process for mass soldering components to both (opposed) surfaces of a circuit board in a single pass.
Still other objects will appear obvious and in part will appear hereinafter.
The invention accordingly comprises the processes involving the several steps and relative order of one or more of such steps with respect to each other and the apparatus possessing the features, properties and relations of elements which are exemplified in the following detailed description and the scope of the application of which will be indicated in the claims.
In the following detailed description of the invention, the term "component" refers to so-called leadless components such as chip components as well as components having conventional metallic conductors or leads. The term "component lead" refers to that part of the metal conductor of an electrical or electronic component that is joined to the printed circuit board pattern, i.e. the component leads, terminals, lugs, pins, et cetera. The term "land" as used herein refers to that part of metallic pattern on the printed circuit board to which a component or component lead is joined by solder. the terms "top side surface" and "bottom side surface" as used herein with reference to the circuit board are meant simply to refer to the opposed broad surfaces of the circuit board; the terms "top" and "bottom" respectively denote spatial orientation of the circuit board only as it is being processed in the soldering apparatus in accordance with the present invention. The term "mass soldering" as used herein is intended to refer to any of the several soldering techniques known in the art in which (a) molten solder is applied to a circuit board from a body of molten solder, including by way of exanple, but not limitation: wave soldering, dip soldering, pot soldering, jet soldering, cascade soldering and drag soldering, or (b) a plurality of solder connections are made essentially simlutaneously by reflow of pre-applied solder preforms or solder cream or paste on a circuit board.
The present invention provides a novel system, i.e. method and apparatus, by which components populating both (opposed) surfaces of a printed circuit board may be mechanically and electrically joined to the printed circuit board by mass soldering in a single pass. More particularly, in accordance with the process aspect of the present invention, a circuit board containing components populating both top and bottom surfaces of the board is first subjected to a first mass soldering operation in which the bottom side surface of the board and components thereon are passed in contact with a body of molten solder as by passing through the crest of a standing solder wave. The first mass soldering operation also imparts substantial heat energy to the board which heat energy is conducted through to the top side surface of the board. Additional heat energy then is supplied to the top side surface of the board to reflow preapplied solder cream or paste or solder preforms on the board top side surface. The apparatus of the present invention comprises a fluxing station wherein flux may be applied to the bottom surface of the circuit board; a preheater station wherein the flux is activated and the board prepared for soldering; a first mass soldering station wherein the bottom side surface of the printed circuit board may be brought into contact with a pool of molten solder; and a second mass soldering station wherein preapplied solder cream or paste or solder preforms on the top side surface of the printed circuit board may be heated to reflow. In a preferred embodiment of the invention the first soldering station comprises a wave soldering assembly consisting of two wave-forming solder sumps including a first nozzle for forming a bidirectional wave and a second nozzle for forming a second substantially unidirectional wave, and the second mass soldering station comprises a convection heating assembly consisting of a pair of heated nozzles for directing heated fluid streams substantially vertically downwardly onto the top side surface of the board. In a particularly preferred embodiment of the invention the heated fluid comprises heated air. Completing the apparatus is means for transporting the circuit boards to be soldered in-line in timed sequence between the first mass soldering station and the second mass soldering (reflow) station.